Papers alleging Auckland mayor Len Brown broke the law by not declaring free hotel rooms have been filed at the Auckland District Court today.

Retired Wellington accountant Graham McCready will bring a private prosecution against the second-term mayor alleging he broke the law by not declaring five free room upgrades from SkyCity for himself and his wife between November 2010 and November 2013.

The SkyCity upgrades were thought to be worth about $4500, McCready said.

McCready said Brown's acceptance of the gifts led to "favourable consideration" towards SkyCity, and he would prosecute the mayor under section 105(1) of the Crimes Act - corruption and bribery of an official.

Brown's record on dealing with SkyCity over its proposed convention centre during the period would make up "part of the prima facie case" against the embattled mayor, McCready said.

The papers were filed this afternoon by activist Penny Bright, a friend of McCready. Bright said she expected McCready to come to Auckland next week to file further documents.

"We only have to show that there is any evidence at all that would establish a prima facie case and we have several of the elements present.," McCready said.

He said it was important Brown be held to account because accepting gifts went right to the heart of the democratic system.

"I think what will come out of this, regardless of the outcome, is that that practice will stop," he said.

McCready has a record of successfully bringing private prosecutions against public figures, including a case against MP Trevor Mallard for fighting in public, and charges against ACT leader John Banks for knowingly filing a false electoral return, which have now been taken over by the solicitor-general.

McCready added he was certain the prosecution against Brown would go "a long way down the track".

"Then who knows. Like the Banks case, the day before he was moved to trial I thought we were goners, but the judges have different views on that, so it's up to him."

Brown will be notified of the filing and will have the opportunity to make his own submissions against the allegations.

Brown's media spokesperson said the mayor had no comment on the private prosecution. Brown is still on holiday.

After Brown's re-election last year, and in the wake of revelations about his affair with council junior Bevan Chuang, auditors found he accepted more than $39,000 worth of hotel freebies over a two-year period.